Recently, developments of electronic paper have been actively performed, as the electronic paper is expected to be an electronic media that replace paper. The electronic paper has a characteristic that a display device is used as if it is paper. Therefore, the electronic paper requires different properties from those of conventional display devices, such as CRT, and a liquid crystal display. For example, the required properties for the electronic paper include being a reflective display device, having high white reflectance and a high contrast ratio, realizing highly precise display, giving a memory effect to a display, realizing low voltage drive, being thin and light, and being inexpensive. As for the properties associated with a quality of a display, white reflectance and a contrast ratio similar to those of paper are highly demanded among the aforementioned properties.
As for a display device for use as an electronic paper, for example, devices of a system using reflective liquid crystals, a system using electrophoresis, or a system using toner migration have been proposed. Among them, the electrophoresis system is a mainstream, and is widely used in electronic paper available on the market as commercial products. However, it is particularly difficult to realize high white reflectance with this method. It has been known that the white reflectance realized in this method is a low value, i.e., about 40%, whereas the white reflectance of paper is 80%, and the white reflectance of newspaper is 60%. Therefore, to realize high white reflectance is a large problem in this system.
As for a promising technique for solving the aforementioned problem, and realizing a reflective display device, there is a system using electrochromic phenomenon. The phenomenon where an oxidation-reduction reaction is reversibly caused to reversibly change a color, as voltage is applied, is called electrochromism. A display device utilizing coloring/discharging (referred as to coloring-discharging, hereinafter) of an electrochromic compound that causes this electrochromic phenomenon is an electrochromic display device. The electrochromic display device is a reflective display device, has a memory effect, and can be driven at low voltage. Therefore, researches and developments of the electrochromic display device have been widely conducted from a development of materials to designing of a device, as a strong candidate for a display device technology for use in electronic paper. For example, it has been confirmed in PTL 1 that white reflectance of an electrochromic display device is 60%, which is substantially the same value to that of paper.
Moreover, the electrochromic display device is a system that can solve most of the aforementioned problems. Since conventional materials for use have symmetric structures, and are disubstituted compounds, however, most of the materials have poor solubility due to a quaternary salt structure, and a phosphoric acid group. In addition, it is difficult to purify the materials to a high degree with a simple operation (see PTL 2 to 3).